A boiler is typically a closed high-pressure system defined by many interconnected headers, pipes, and tubes and containing a fluid that can be heated under controlled conditions. As the fluid is heated to a certain temperature, the fluid absorbs energy. This fluid can then be used to provide work, or it can be used as a source of heat.
Fuel used to heat the fluid in the boiler is burned in a furnace portion of the boiler. In a boiler that employs water as the fluid contained therein, waterwalls are positioned around the furnace and contain tubes through which the fluid flows. The typically deaerated fluid is first fed to tubes of an economizer and then is fed to the tubes in the waterwalls. The economizer receives feedwater and makeup water, which replaces losses from the steam produced. The economizer absorbs heat from flue gases produced from the burning of fuel in the furnace and transfers the heat to the feedwater and the makeup water.
In a supercritical boiler, fluid from the economizer is converted to steam as it passes through the tubes in the waterwalls. The steam may be used directly in a process (to produce work or as a source of heat). If not used directly in a process, the steam may be passed to a superheater wherein the steam is heated further. The superheated steam increases the efficiency of a steam turbine to which it is supplied.
Typically, the temperature of the boiler flue gas leaving the economizer is lower when the boiler is operating at reduced steam flows. In instances when the boiler operates with a selective catalyst reduction (SCR) system at the flue gas exhaust, the reactiveness of the catalyst is dependent upon the flue gas temperature entering the catalyst reactor. Accordingly, a reduction in flue gas temperature below a threshold value results in the catalyst being less reactive.